The present invention relates to aqueous heat-curable compositions which are based on known heat-curable condensation products of aldehydes and hydroxy-and/or amine containing compounds. The invention also relates to the use of such compositions as glues, as impregnating materials for cellulosic webs and for preparing moldings.
Curable condensation products of aldehydes and hydroxy-and/or amine containing compounds are of great industrial importance. They are precondensates which may contain various additives and which are used for various purposes, in particular as glues and as impregnating materials for cellulosic webs which are then turned into surface coatings. The condensation products are preferably phenol-, resorcinol-, urea- or melamine-formaldehyde resins. They are generally used in the form of aqueous solutions or emulsions. When these aqueous solutions or emulsions are used as glues, the water is absorbed by the somewhat absorbent materials to be bonded to each other.
When used as impregnating agents, curing is generally preceded by drying. The curing is effected at elevated temperatures. Depending on the intended use, the condensation products could contain catalysts, for example alkalis, acids, metal salts, plasticizers, surface-active substances and film-forming substances, for example polyvinyl alcohol or polyvinyl acetate.
Possible fillers are coconut shell powder, bark powder, walnut shell powder, stone powders, chalk, wood flour or other customary fillers.
Possible extenders are blood albumin, rye, wheat, bean or potato flours, or other customary extenders.
The addition of these agents reduces the consumption of the actual glue resins, improves the flow properties of glue line and holds back the glue resin in the glued joint. Excessively high additions of the agents reduce the water resistance of glued joints.
It is also known that starches which have been cooked, for example treated with alkali under heat, or additionally modified starches produce, on admixture of durosetting plastics, starches which combine substantial joint strength with substantial water resistance. For instance, British Pat. No. 527,704 describes the preparation, from melamine-aldehyde condensation products and starch conversion products, of glues which are water-soluble or which, stirred with cold water, give a paste. Hot-pressed test specimens of glues containing 70-89% of modified starch and 20% of plastic had very strong and water-resistant glued joints.
On increasing the modified starch content to 90-95% and, correspondingly, reducing the plastic content to 10-5% the glued joints were found to be still stronger: loading did not break the glued joint but the wood. However, the water-resistance of these samples was lower.
According to U.S. Pat. No. 2,275,314, cooked starches, for example--predominantly cassava flour which has been cooked in an alkaline medium--are added in preferred amounts of 40-60% of urea-formaldehyde condensates. On hot-pressing, the resulting glues displayed equal or improved dry strengths and, usually, significantly improved wet strengths compared with starch and casein glues of the type used for glueing wood.
By reacting sucrose with phenol and formaldehyde in alkaline solution, it is possible to obtain boilproof phenolic resins in which about half the phenol is replaced by sucrose (C. D. Chang and O. K. Kononenko, Adhesives Age, 1962, 36-40).
In Canadian Pat. No. 1,090,026, phenol, urea and carbohydrates, such as dextrin or starch, are condensed under acid conditions. The resulting products were processed together with commercially available fillers and curing agents into compression-molding compositions which satisfy the requirements of ASTM D570-63 (6a). the condensates were also further condensed with formaldehyde under alkaline conditions to give resols which gave boil-proof glueings.